The present invention relates to clay-free aqueous well drilling and servicing fluids, methods of preparation thereof, and method of drilling a well therewith.
The use of fluids for conducting various operations in the boreholes of subterranean oil and gas wells which contact a producing formation are well known. Thus drill-in fluids are utilized when initially drilling into producing formations. Completion fluids are utilized when conducting various completion operations in the producing formations. Workover fluids are utilized when conducting workover operations of previously completed wells.
One of the most important functions of these fluids is to seal off the face of the wellbore so that the fluid is not lost to the formation. Ideally this is accomplished by depositing a filter cake of the solids in the fluid over the surface of the borehole without any loss of solids to the formation. In other words, the solids in the fluid bridge over the formation pores rather than permanently plugging the pores. This is particularly critical in conducting horizontal drilling operations within hydrocarbon containing formations.
Many clay-free fluids have been proposed for contacting the producing zone of oil and gas wells. See for example the following U.S. Patents: Jackson et al. U.S. Pat. No. 3,785,438; Alexander U.S. Pat. No. 3,872,018; Fischer et al. U.S. Pat. No. 3,882,029; Walker U.S. Pat. No. 3,956,141; Smithey U.S. Pat. No. 3,986,964; Jackson et al. U.S. Pat. No. 4,003,838; Mondshine U.S. Pat. No. 4,175,042; Mondshine U.S. Pat. No. 4,186,803; Mondshine U.S. Pat. No. 4,369,843; Mondshine U.S. Pat. No. 4,620,596; Dobson, Jr. et al. U.S. Pat. No. 4,822,500, and Johnson U.S. Pat. No. 5,504,062.
These fluids generally contain polymeric viscosifiers such as certain polysaccharides or polysaccharide derivatives, polymeric fluid loss control additives such as lignosulfonates, polysaccharides or polysaccharide derivatives, and bridging solids. As disclosed in Dobson, Jr. et al. U.S. Pat. No. 4,822,500, a xanthan biopolymer and an epichlorohydrin crosslinked hydroxypropyl starch fluid loss control additive synergistically interact to provide suspension and fluid loss control in certain of these fluids.
Magnesium oxide has been disclosed for use in various polysaccharide-containing fluids to increase the thermal stability thereof. See for example the following U.S. patents: Jackson U.S. Pat. No. 3,852,201; Jackson U.S. Pat. No. 3,953,335; Hartfield U.S. Pat. No. 3,988,246; Jackson U.S. Pat. No. 4,025,443; and Dobson, Jr. U.S. Pat. No. 5,514,644.
It is well known that certain biopolymer-containing fluids are shear thinning, exhibiting a high low shear rate viscosity and a low high shear rate viscosity. A near zero shear rate (0.06 to 0.11 sec.sup.-1) viscosity provides a numerical value related to the ability of a fluid to suspend particles or cuttings under static conditions. Conversely, viscosity measured at shear rates above 20 sec.sup.-1 relates to the hole cleaning capability of a fluid under annular flow conditions.
It is known to use dense brines as the base aqueous liquid for high density drilling and well servicing fluids. Such fluids contain minimal soluble bridging solids concentration and viscosifying polymer additives. The most commercially available dense brines contain calcium chloride, calcium bromide, and zinc bromide. However, utilization of the brines has been limited. Generally, water-soluble polymers used for viscosity and filtration control do not perform well in calcium bromide and zinc bromide brines. Examples of the use of dense brines for use in well drilling and servicing fluids are as follows: Swartwout et al. U.S. Pat. No. 5,612,293; Dobson, Jr. et al. U.S. Pat. No. 5,616,541; Dobson, Jr. et al. U.S. Pat. No. 5,728,652; Dobson, Jr. et al. U.S. Pat. No. 5,728,654; Vollmer et al. U.S. Pat. No. 5,785,747; and Dobson, Jr. et al. U.S. Pat. No. 5,804,535.
Clean-up of the filter cake deposited on the sides of the borehole is a critical part of the completion process to ensure maximum productivity from a wellbore. Poor wall cake development allows particulates or liquids to enter the formation resulting in internal formation damage. Solids or polymers which have not been removed from the surface of a borehole can also impede the flow of hydrocarbons by plugging a prepacked screen during production. Filter cake removal is generally undertaken by treating the wellbore with concentrated acid solution, particularly hydrochloric acid. Dobson, Jr. et al. U.S. Pat. No. 5,607,905 discloses the incorporation of certain inorganic peroxides into the filter cake which enhance the removal of the filter cake upon contacting it with an acidic solution. Brannon et al. U.S. Pat. No. 5,881,813 discloses an enzyme-containing clean-up fluid for degrading the residual polymeric viscosifiers present in filter cakes from drill-in fluids or present in the formation from other well treating fluids.
While these improvements in formulating well drilling and servicing fluids from high density brines have had commercial success, there is still a need for such fluids which exhibit enhanced particulate suspension characteristics at lower viscosities and which are easier and more completely removed from wellbores, screens, and the like present in hydrocarbon-containing formations.